Metal–Metal Quadruple Bonds Supported by 5‑Ethynylthiophene-2-carboxylato Ligands: Preparation, Molecular and Electronic Structures, Photoexcited State Dynamics, and Application as Molecular Synthons

From the reaction between M2(TiPB)4 and 2 equiv of 5-ethynyl­thiophene-2-carboxylic acid (H-ThCCH) in toluene, the complexes trans-M2(TiPB)2(ThCCH)2, where M = Mo (I) or W (II) and TiPB = 2,4,6-triisopropyl benzoate, have been isolated and characterized by 1H NMR, IR, MALDI-TOF MS, UV–vis, steady-state emission, transient absorption, and time-resolved infrared (TRIR) spectroscopies and single-crystal X-ray crystallography for I. The molecular structure of I confirms the trans-substitution pattern and the extended conjugation of the ethynyl­thienyl ligands via interaction with the Mo2δ orbital. The HOMO of both I and II is the M2δ orbital, and the intense color of the compounds (I is red and II is blue) is due to the M2δ-to-ThCCH 1MLCT transition. The S1 states for I and II are 1MLCT. The T1 state is 3MLCT for II, but 3MoMoδδ* for I. The TRIR spectra of the ν­(CC) stretch in the MLCT states are consistent with the delocalization of the electron over both ThCCH ligands. Compound I is shown to be a synthon for the preparation of trans-Mo2(TiPB)2(ThCCPh)2 (III) and trans-Mo2(TiPB)2(ThCCAuPPh3)2 (IV). Both III and IV have been characterized spectroscopically and by single-crystal X-ray diffraction. The structure of III indicates the extended π-conjugation of the trans-ethynyl-thienyl units extends to the added phenyl rings.